Exploration Geophysics - Volume 24, Issue 3-4, 1993

Interpretation of new seismic, gravity and aeromagnetic data sets from the western Otway Basin has contributed significantly towards a better understanding of the regional tectonic framework of the basin’s evolution. Geosat data from the Southern Ocean and gravity data onshore have defined what is here termed the Otway-Sorell microplate, a triangular region of extended continental crust and lithosphere bounded by the onshore northern limits of the Otway Basin, the eastern limits of the Sorell Basin, and a prominent Geosat lineament (?fault zone) extending from the offshore Crayfish Platform to southern Tasmania. The Geosat data also redefine the Spencer and Tasman Fracture Systems within the Southern Ocean lithosphere, and the existence of the previously named Gambier, St. Vincent and George V fracture zones is now doubtful.

Fragmentation of Australia’s continental crust during Jurassic lithospheric extension led to the development of a series of rift segments and intervening crustal blocks in the western Otway Basin. Long wavelength aeromagnetic anomalies outline what Is interpreted as the extent of Palaeozoic crustal fragments and a possible lithospheric triple junction, including the newly-named Coorrong Trough. This trough is along strike from the Scopes Range — Padthaway Gravity High extending south from the western margin of the Murray Basin, suggesting a significant onshore-offshore structural correlation near the traditional Precambrian-Phanerozoic boundary, known as the Tasman Line. The boundaries of Palaeozoic crustal fragments can be correlated with significant changes in the character of deep seismic profiling data and a thinning of the crust south of the Tartwaup Fault Zone. The Tartwaup fault zone is interpreted as the cratonic boundary for a lower-plate margin during the episode of lithospheric extension which eventually resulted in separation of Australia from Antarctica.

Seismic mapping of the Otway Group sequences defines the early rift segments and the regional scale of later deposition. The earliest rifting developed in a series of half graben with most of the major bounding faults dipping towards the craton. These rift segments did not develop fully. However, rifting southwest of the Tartwaup fault zone did progress to the stage of continental separation. There are significant northwest and northeast structural trends which can also be identified in magnetic and gravity images. Fault reactivation occurred in Aptian, Late Albian, Late Maastrichtian, Eocene and Miocene times in response to intra-plate stress being transmitted from plate boundaries.

The advent of small, cheap, and highly portable Global Positioning Systems (GPS), together with recent advances in ground magnetometer technology, has allowed the development of an economic GPS positioned ground magnetic surveying system. Like their larger sophisticated counterparts in the aeromagnetic arena, these systems facilitate the collection of magnetic data without the need for any navigation tool other than the GPS. Unlike their counterparts, however, they may be constructed at minimal cost, with minimal research and development, and with a very short lead time.

A simple marriage of off the shelf components has resulted in the creation of POSMAG, a GPS positioned ground magnetic system. POSMAG can acquire magnetic and positional information in real time. It is based on a Garmin GPS unit and a Gem GSM- 19f Overhauser magnetometer. Data is collected in a “hands free” operation, at normal walking speeds, with high along line spatial resolution, and on arbitrarily placed survey lines. GPS data is corrected in a pseudorange differential fashion with post processing and combined with the magnetic data on a time basis. Positional accuracies averaging 8 m have been achieved.

The system is used in all reconnaissance applications, particularly in rugged areas where grids are prohibitively expensive to establish. High along line resolution, and speed of operation make POSMAG a cheap and effective tool. Areas previously not amenable to assessment with the ground magnetic method are now covered effectively and economically.

This paper describes the features and limitations of the equipment used in the POSMAG system. Results of tests on the accuracy of GPS positioning with rudimentary post processing are given. Finally, case studies are presented to illustrate the uses of the system in both standard grid and reconnaissance applications.

The purpose of this paper is to demonstrate 3-D prestack depth migration as an ultimate technique for imaging structural targets associated with salt diapirism, and compare it with other migration strategies — 2-D prestack, and 2-D/3-D poststack depth migrations. The structural problem is to image sedimentary bed terminations, bottom salt boundaries and undersalt layers around two large salt masses in the Gulf of Mexico. A large number of oil and gas reservoirs in the Gulf of Mexico is controlled by salt structures. Typical salt flank plays have been mapped quite successfully with time migration strategy. However, as exploration moves into more complicated targets around salt overhangs and subsalt traps, depth migration strategy becomes necessary.

The first task is to build a detailed 3-D velocity-depth model. To accomplish this, the following robust approach has been chosen. Using a velocity-depth model for the overburden (sedimentary section) based on edited stacking velocities, 3-D poststack depth migration was performed to verify its accuracy. Iterative 3-D poststack depth migration then followed to define the salt masses. Top salt reflectors were interpreted in 3-D and constant salt velocity was assigned to the half-space below the top-salt. Again, 3-D poststack depth migration was performed and base-salt events were interpreted. Finally, the velocity model was updated to contain complete salt bodies and 3-D poststack migration was rerun to check results. This loop of three iterations was repeated until convergence was achieved — the input model matched the result of the depth migration.

Given the velocity model, a single output line was selected and 3-D dynamic raytracing was performed to link output image points with surface shot-geophone locations. Finally, 3-D Kirchhoff depth migration was run in 3-D prestack, 2-D prestack and 2-D poststack modes to produce comparison sections along the output line.

Analysis of regional aeromagnetic and gravity data sets shows that the Stuart Shelf is host to a major volcano-plutonic complex consisting of large volumes of felsic-dominated volcanic rocks and shallow-crustal level granitic bodies. The complex is interpreted as a major eruptive centre associated with the Gawler Range volcano-plutonic event of 1600 Ma to 1580 Ma. The mid-Proterozoic basement surface in the northern and central areas of the Stuart Shelf lies beneath a thick cover of younger sediments and is interpreted as representing a section through the lower-middle levels of the volcano-plutonic complex. Volcano-plutonic elements present at this stratigraphic level include collapsed cauldrons, epizonal granites, and ring faults associated with pluton emplacement. Some of these structures acted as fluid pathways during an extensive period of Fe-metasomatism associated with the volcano-plutonic event, and consequently are evident in the geophysical data as gravity and/or magnetic highs resulting from localised deposits of magnetite and hematite. The Stuart Shelf is a prime example of a terrane where the geophysical response is dominated by secondary alteration assemblages rather than primary lithologies. The presence of a thick cover sequence over the entire Stuart Shelf restricts direct examination of the volcano-plutonic complex. However, comparison of the geophysical data from the Stuart Shelf with data from well-exposed, well-documented terranes of a similar tectonic setting provides a useful insight into the possible structural and tectonic history of the Stuart Shelf. Such terranes include the mid-Proterozoic St. Francois Mountains of south-east Missouri and the Quaternary volcanic provinces of the mid-western United States of America.

Conventional methods of treating boundaries in finite difference (f, x) migration, namely padding in both space and time, have the disadvantage of increasing the data volume. This increase, whilst acceptable for 2-D processing, becomes uneconomical in the 3-D arena. Dipping events move up-dip during the migration process. Dips close to the sides of the data that move outside the data range can be reflected back if the boundary conditions are poorly set. This results in misleading events appearing in the migrated data. The addition of extra zero trace padding allows these events to move out of the data and so avoids these artefacts.

The alternative to padding with zero traces is to use absorbing side boundary conditions. Absorbing sides have been proposed by several authors. A method developed by Clayton and Engquist is reviewed in detail, and implementation issues are discussed. Although this method is exact for only one propagation angle, it is found to attenuate significantly the reflections of other dipping events.

Time padding is used to avoid the wraparound that causes shallow data to reappear as noise at later times in the migrated result. Wraparound is inherent in (f, x) finite difference migration, because of the cyclic nature of the fast Fourier transform. The time padding increases the run time of both the fast Fourier transform and the migration.

Clearly, it is desirable to find a technique to remove the wraparound and so avoid the need for costly time padding. Kjartansson showed a method for wraparound removal for the case when the sampling interval is equal to the migration time step or depth interval. In practice, however, a coarser time step is usually employed. A new generalised method has been developed for wraparound removal under these conditions of coarse time step. The technique is applied at each time step extrapolation. Although this obviously involves more computation, the method is more efficient than using extra time padding.

Both these techniques, the absorbing side boundaries and the wraparound removal, can be used in 2-D and one-pass 3-D implementations. Without the need for extra padding, migration run times and hence costs are reduced. The methods are demonstrated on synthetic data. The results compare well with those obtained using conventional padding, with a considerable reduction in run time.

3-D marine seismic surveys acquire ever larger volumes of data, and use increasingly complex deployments of multiple streamers and source arrays to reduce costs and increase efficiency. The result of such large volumes of data presents serious problems of acquisition quality control and processing turnaround.

The rapid development of on-board processing systems has enabled these problems to be addressed directly, by providing comprehensive features for quality control, smoothing the path for final data processing, and providing access to 3-D data volumes at an early stage. Apart from the processing power provided by the modern workstation computer, the key elements in this development are the availability of real-time data links with the recording system, and the provision of similar linkages to the real-time and post-processed navigation data. These linkages enable real-time processing of a significant subset of the 3-D data in the form of a stack of the near offset traces (typically 6- or 12-fold) of all subsurface lines recorded simultaneously in multiple source and streamer acquisition. The stack traces are assigned appropriate mid-point co-ordinates and stored in the form of a 3-D volume.

The 3-D data may be viewed as in-line, cross-line or time-slice profiles at any time during the survey (and the processing), which allows the correct functioning of many key acquisition sub-systems to be verified.

The 3-D volume may be migrated on-board, or soon after completing the acquisition of the date, allowing rapid access to 3-D migrated results, and providing a basis for initial interpretation and velocity model building. The on-board processing system thus brings together all aspects of the 3-D survey — acquisition, processing and interpretation.

Woodside has conducted two high resolution aeromagnetic surveys in the northern and southern extremities of the Vlaming Sub-basin. The northern survey also covered a portion of the Edwards Island Block and the southern extremity of the Abrolhos Sub-basin. The acquisition of the data highlighted the problem and characteristics of ocean swell generated magnetic noise, which had amplitudes similar to the structurally related signal.

The aeromagnetic data defined the broad basin architecture. Image processing helped to enhance many of the weaker magnetic features which correlated with intra-sedimentary structures observed on the seismic data. The aeromagnetic data showed that the structuring and the structural orientations are slightly different between the two survey areas. The northern area had a wider envelope of fault azimuths with trends of NW to NNE with the NW to NNW trends dominant. The southern area had a narrower envelope of fault azimuths with trends predominantly NNW to NNE.

In the northern area NW lineaments, parallel with the oceanic transforms, offset other lineations in a right-lateral direction. These are interpreted to be right lateral strike-slip zones related to oblique faulting during breakup. Similar NNW strike-slip features are observed in the southern survey data set. East-west lineaments are interpreted as antithetic strike-slip faults. These are believe to be related to basement block rotations which caused localised areas of compression, observed on seismic data.

The aeromagnetic and seismic data show the Vlaming Sub-basin underwent right lateral oblique faulting during the breakup between Australia and India. Changes in fault drag on the Darling Fault, possibly related to variations in basement between the two survey areas, are proposed as the reason for the differences in fault azimuths.

In prediction of reservoir properties using seismic data, use is frequently made of AVO techniques. However, such techniques may not always be successful. Hence a new technique has been devised — based on recording a subsurface AVO response in the well. Recording is achieved from five triaxial geophone shuttles at 15 m intervals — ideally one wavelength (of the predicted dominant source frequency) above the target. The source (two 150 cubic-inch Sleeve Guns) was deployed from the marine vessel’s boom and moving in a line such that shot spacing is as close as possible to the surface seismic offsets. The line might extend 3 km to each side the well. Processing of the AVO walkaway data takes place in three steps: data orientation; separation of the down and up P-wavefields; and deconvolution (using the downgoing P-wavefield to deconvolve the upgoing P-wavefield) to remove propagation effects. The surface seismic data (in this case a 3-D data set) are processed (gathered) and put through a rigorous 12-stage signal processing sequence. The AVOWSP and the surface gathers are then carefully tied to the zero-offset VSP. After application of the Offset Dependent Gain Function the data are solved for P-wave reflection amplitude (at angle 0°) and the gradient. Those results can then be mapped and contoured — to identify anomalous areas, at known sites (wells) and elsewhere. Additional refinement is continuously being sought, and is welcome.

The performance of three post-stack seismic inversion techniques has been evaluated using synthetic and VSP data. The application of classical recursive inversion to simple synthetic models demonstrates the importance of low frequencies in trace inversion. This is consistent with the strongly low-pass response of the inversion transfer function. Impedance profiles generated from band-limited seismic traces contain only “depth-positional” information. An accurate impedance profile can only be obtained if independent, reliable low-frequency information is incorporated following inversion. Autoregressive (AR) spectral extension is evaluated as a representative of sparse-spike inversion techniques. Synthetic examples suggest excellent performance on sparse-spike models, but performance deteriorates as the reflectivity series becomes more dense. Poor performance on real VSP data is a further indication of the failure of the AR technique when fundamental assumptions are not satisfied. Of the three methods investigated, model-based linear inversion is preferred in terms of accuracy, robustness and logistics. In contrast to previously published modelling algorithms, no geological interpretation is required prior to inversion. Instead, the inverted band-limited trace is used to initiate the inversion scheme. Critical low-frequency control is achieved by incorporation of RMS velocity constraints. The robustness of the model-based inversion technique when applied to VSP data suggests that it is possible to recover accurate impedance information beyond the bottom of the borehole.

If it is assumed that sonic velocity in a sedimentary rock decreases with burial-depth according to a known velocity/depth relationship, and that velocity is not reduced by erosion from maximum burial-depth, any erosion of that rock from maximum burial-depth may be quantified using velocity data. The displacement, on the depth axis, of sonic velocity in a given unit from the normally compacted velocity/depth relationship yields apparent erosion (i.e. amount of missing section or height above maximum burial-depth).

Shales have been considered to be the only lithology to follow a sufficiently predictable velocity/depth relationship with burial-depth to be used in the estimation of erosion from sonic velocity data. However, erosion estimates based on sonic velocities in the Lower Cretaceous Allaru/Oodnadatta Mudstone of the Eromanga Basin are statistically similar to those derived from velocities in the Middle Jurassic Hutton Sandstone (an important hydrocarbon reservoir sandstone in the basin). Similarly, erosion estimates based on velocities in the Lower Triassic Bunter Sandstone (an important hydrocarbon reservoir sandstone) of the United Kingdom Southern North Sea are statistically similar to those based on velocities from the Lower Triassic Bunter Shale.

The consistency of results from the shaly and sandy units analysed suggests that overcompaction (i.e. anomalously fast sonic velocity) of the sandstones is controlled by erosion from previously greater burial-depth, rather than by burial-depth independent sedimentological and/or diagenetic processes. The results validate the use of sandstones in maximum burial-depth studies, and perhaps more importantly suggest that, even in reservoir sandstones, burial-depth is the primary control on compaction and hence porosity.

The interpretation of borehole breakouts suggests that minimum horizontal stress (σh) Is regionally oriented approximately N-S in the Barrow-Dampier Sub-Basin. More locally, breakout interpretation suggests that σh orientation is 005°-010° in the Wanaea/Cossack area, and 012°-022° in the Griffin/Chinook/Scindian area. Horizontal stress magnitudes in the Wanaea/Cossack area, derived from modified leak-off tests, together with vertical stress magnitudes derived from density and sonic log data, suggest that the stress (fault) regime in the Wanaea/Cossack area is on the boundary of extension (normal faulting) and strike-slip.

The contemporary stress field impacts on planning the drilling direction of deviated and horizontal wells through the issues of mechanical wellbore stability and fracture intersection. In order to maximise intersection with any open, natural fractures, and to optimise any hydraulic fracturing program, wellbores should be horizontal and parallel to σh (minimum horizontal compressive stress) in the extensional (normal fault) and strike-slip stress regimes, and vertical in the compressional (reverse fault) regime. In order to minimise the tendency for borehole breakout, the stress anisotropy around the wellbore should be minimised — i.e. horizontal wells should be drilled in the direction of σh in the extensional regime, and in the direction of σH (maximum horizontal compressive stress) in the compressional regime. In the strike-slip regime, horizontal wells can be oriented such that they are not subject to any stess anisotropy by progressively changing from the σh direction towards the σH direction as the horizontal stresses increase with respect to the vertical.

In the Wanaea/Cossack area, wells drilled towards 005°-010° or 185°-190° will maximise the potential for intersection with open, natural fractures (should they be present), and recovery from any induced, hydraulic fracturing will be optimised. Furthermore, wells in this direction will be subject to the minimum stress anisotropy, and are thus least likely to be subject to breakout.

A test survey using the magnetic induced polarization (MIP) method was executed over the HYC sulfide deposit in the McArthur River area of the Northern Territory, Australia in September, 1992. The survey conducted by the Metal Mining Agency of Japan (MMAJ) using Scintrex equipment and personnel, was planned to define the MIP response over the huge stratified sulphide deposit and to compare the results with other geophysical data (including conventional IP) compiled since discovery in 1955.

For current flow parallel to strike of the HYC deposit, a strong MIP anomaly was observed, providing accurate interpretation of depth and dip direction of the ore horizon and the upper pyritic shale-siltstone member. The MIP anomaly was in good agreement with the conventional electrical IP data collected with pole-dipole array.

On the other hand, current flow perpendicular to strike produced no MIP anomaly. This result is consistent with the theory of MIP, indicating that the exploration programme must take account of regional geology and structures.

Petrophysical study of drillcores showed the strongest IP source to be the ore horizon containing abundant sulphides. The upper pyritic shale-siltstone member also contains a considerable amount of sulphide and gives a significant IP effect.

The MIP test survey verified the HYC orebody as a target for both electrical and magnetic IP methods, and MIP is suggested as a viable prospecting technique for similar base-metal targets in the region.

Analyses of fault trends and aeromagnetic images, together with burial history modelling, show that the onshore northern Perth Basin has a complex tectonic history. Three major phases of tectonism are recognised.

1) Extension in a direction of 005° in the Late Permian that resulted in normal faults striking 275°, and sinistral strike-slip along the Darling Fault.

2) Extension in a direction of 355° in the Jurassic resulted in the onset of rifting which produced normal faults striking 265°, and sinistral strike-slip on transcurrent faults oriented at 310°.

3) The break-up of Greater India from Australia in the Early Cretaceous and the resultant extension direction of 255° produced normal faults striking 345°, and dextral strike-slip along transcurrent faults oriented at 300°.

Japan National Oil Corporation (JNOC) acquired the following shear wave seismic data around the TRC-AMR1 (JNOC research well) in the Amarume oil field which is located in the northern part of Japan:

1. 1989 Shear wave VSP by a single, coupled source
2. 1990 Shear wave surface seismic by a single, coupled source
3. 1992 Shear wave VSP by dual coupled sources

Rotation analysis for VSP data of the first survey showed remarkable shear wave splitting. The surface seismic data of the second survey did not show this effect.

In both surveys, the motion of the shear wave source was limited to a direction perpendicular to the source-receiver line. In general, rotation analysis requires data from dual coupled sources whose motions are perpendicular to each other. The last VSP data (3) were acquired in order to investigate the conflicting results of both surveys.

Data from this survey confirmed the existence of an anisotropic layer shallower than 700 m.

The direction of anisotropy was found to be almost east-west, that is, in line with the receiver direction of the second survey. It was concluded that this was the reason why the second survey did not show shear wave splitting. The time difference between S1-wave (the faster shear wave) and S2-wave (the slower one) was over 100 ms at 700 m depth.

This survey also detected the location of a gas reservoir at around 870 m depth.

This paper presents a technique for the integrated forward modelling of the structure and geophysical response of multiply deformed terranes. This technique allows information collected by field geologists and geophysicists to be reconciled by the development of a simplified structural history of the area. The modelling is based on the deformation history of the area, in terms of a succession of structural events, such as folds, shear zones and intrusions. The interaction of these events with an assumed stratigraphic model results in the prediction of feasible structures. By specifying rock properties for the units in the initial stratigraphy, predictions can also be made as to the potential field anomalies for gravity and magnetics. The accuracy of the model can be gauged by comparing the predictions with the constraints provided by the observed structural and geophysical data. This approach points to a new methodology for the reconstruction of the geometry of structures in the Earth’s crust, and has potential as a tool for both research and training.

Energy spectral analysis applied to regional aeromagnetic data covering an area between 26°S and 35°S and 129°E and 141°E indicates an interface which lies at depths of between 5 km and 25 km. Structures indicated on the contour map of the depth of this interface are related to major features on the gravity map and to folds and faults in the Musgrave Block, the Gawler Craton and the Adelaide Fold Belt as well as to kimberlite intrusions in the Adelaide Fold Belt.

Palaeomagnetic orientation of drill core from the Sydney-Bowen Basin is feasible. A consistent magnetisation is observed in coal measures in the basin enabling the remanence of an unoriented sample to be utilised to orient the sample. Palaeomagnetic orientation of drill core for fracture analysis has been used successfully in the Sydney-Bowen Basin.

The direction of magnetic remanence in the sediments of the southern Sydney Basin is consistent across a wide area, this direction being north and up. Similarly, the remanence direction observed in Bowen Basin coal measures is consistent from the south (Moura mine) to the north (Goonyella mine) of the basin, and this direction is also north and up. The consistency of the remanence direction is lithology dependent. Fine sandstone and siltstone/mudstone units gave the most reliable results. Coarse sandstones gave inconsistent remanence directions and should not routinely be used for drill core orientation using palaeomagnetism.

A longcore magnetometer constructed by the CSIRO Rock Magnetism Group enables measurement of the remanence of HQ drill core without the need to subsample. The measurement of the remanence of HQ drill core using a longcore magnetometer shows similar results to the remanence measured using laboratory magnetometers indicating that drilling does not alter the NRM. Thus, the noninvasive orientation of drill core by measurement of remanence using a longcore magnetometer is feasible for samples from the Permian coal measures of the Sydney-Bowen Basin. As well, storage of the drill core has no or limited effect on the remanence of the drill core. A small component is sometimes present but is easily removed by AF demagnetisation.

Weathering has a severe effect on the remanence of samples, inhibiting orientation by palaeomagnetism. Weathering can produce hematite/goethite which retains a strong chemical remanent magnetisation and thus dominates the NRM of the sample.

It was anticipated that the altered rock mass associated with Pb-Zn mineralization within Proterozoic dolomitic siltstones at Hilton, NW Queensland, would create problems during mine development. The material is variably oxidised, very porous, saturated and extensive. Semi-regional gravity data have been used to estimate the volume and extent of such material and assist mine planning and dewatering studies. Interactive, segmented, three dimensional procedures controlled by exploration drilling were used. The interpretation considers the effect of all materials and is not restricted to the high contrast alteration. The altered rock mass was found to be irregular, dip steeply west and to be etched from the ore-bearing horizons. It is up to 400 m deep. The alteration has been controlled by the steeply dipping form of the host rock footwall as well as by local faults and jointing. The shape of the cone of depression, after dewatering for more than five years, has confirmed the interpretation within the first development block and lends confidence to the more regional implications along a strike length of at least 6 km. Analyis has also indicated that the volume and density contrast of the ore zone also varies along strike. The average bulk densities deduced for the ore and alteration zones are 3.1 gm/cm³ and 2.1 gm/cm³ or a contrast of about +0.4 gm/cm³ and −0.6 gm/cm³ with respect to the Mount Isa Group host sequence.

AVO (Amplitude Versus Offset) is the seismic technique used for mapping lithology, and modelling is an important step for successful AVO interpretations. Shear velocity measurements are essential, since AVO attempts to exploit the elastic (as opposed to acoustic) nature of seismic wave propagation. A property of seismic wave propagation not often considered is anisotropy. This is probable because the magnitude of the anisotropy has been difficult to measure, and its effect on AVO is not widely known.

New technology is helping to improve AVO modelling. Dipole source shear logging tools can now measure very slow shear velocities, increasing the range of applicability of AVO, and new borehole seismic techniques can measure anisotropy. When integrated, these new measurements provide more detailed information about the elastic moduli that govern wave propagation, and bring the possiblity for greater reliability in AVO interpretation.

The effect of anisotropy on AVO is found to be significant and may lead to misinterpretations of AVO anomalies.